This invention relates to an improvement in a system for detecting and transmitting instant graphic information, such as cursive writing, as the graphic information is developed with a stylus on a table, and more particularly to an improvement in the stylus used to develop the graph and instantaneously pick up position signals for transmission to a receiver where the graph is reproduced in response to the signals.
In a typical system of this type, a grid of conductors embedded in a writing table are energized with very low pulse currents at distinct frequencies for the two axes, and distinct phase (time intervals) along each axis. This causes distinct electrostatic fields that vary in phase with position along the distinct axis. A tablet of paper placed on this table is used to develop the graphic information by an operator using a stylus which is designed to detect the electrostatic fields at all positions through which the stylus is moved on the tablet by the operator. Such a stylus is disclosed by the present inventors Alex Marcel Muller and Joseph Leslie Tyrrell, in U.S. Pat. No. 4,034,155. In that stylus, a ballpoint pen cartridge inside a pen housing is surrounded over a substantial portion of its length by a tubular member electrically shielded from the outside electrostatic field by the pen housing. Only the pen point is unshielded, and therefore exposed to the electrostatic field, to pick up position signals that are then conducted through the entire length of the pen cartridge and capacitively coupled into the tubular member. The latter is connected to a circuit contained within the housing for signal detection and preamplification.
Since the position signals must be capacitively coupled into the pen point, it has been found that the condition of the tablet directly under the pen point will affect the quality of the position signals picked up. As might be expected, the greater the number of sheets of paper in the tablet, the poorer the quality, but that has not presented any serious problem since it has been found that a stack 3/4" high of dry paper may be placed on the table and the stylus is still able to pick up detectable position signals. The problem has been when the paper is not dry due to absorption of moisture in the atmosphere while the tablets are in storage, transit or actual use. The quality of the signal that is detectable for a given stack height may become too undetectable for reliable transmission, depending upon the amount of moisture present in the paper. Accordingly, it is a general object of the present invention to improve the reliability of the signals detected for transmission from an electrostatic field table by a stylus.